lang: rename signed integer types sN -> iN

Surface rename of the signed integer family: s1..s64 become i1..i64
(u1..u64, usize, isize unchanged). 'string' keeps the s-prefix arm in
name classification; width parsing moves to the i-prefix arm next to
isize.

Internal TypeId tags follow the surface (.s8/.s16/.s32/.s64 ->
.i8/.i16/.i32/.i64), as do mono-key mangle fragments (ptr_i64,
tu_i64_bool) and all display/diagnostic formatting (i{d}).

Migrated in the same sweep: stdlib + examples + issue repros + FFI C
companions (shared symbol names like ffi_id_i64), expected
stdout/stderr/ir snapshots, specs.md, readme.md, CLAUDE.md/AGENTS.md,
implementation_plan.md, docs/, issue writeups. Vendored stb_image and
historical flow state left untouched.

zig build test: 426/426; examples suite: 595/595.

examples/1419-ffi-jni-class-09-multi-float-args.sx

@@ -16,12 +16,12 @@
 #import "modules/std.sx";
 
 MotionEvent :: #foreign #jni_class("android/view/MotionEvent") {
-    getAction :: (self: *Self) -> s32;
+    getAction :: (self: *Self) -> i32;
     getX      :: (self: *Self) -> f32;
     getY      :: (self: *Self) -> f32;
 }
 
-sx_consume_touch :: (action: s32, x: f32, y: f32) {
+sx_consume_touch :: (action: i32, x: f32, y: f32) {
     // Black-hole call so the args aren't dead-stripped before LLVM
     // verification gets a chance to look at the call site.
     if action == 0 and x == 0.0 and y == 0.0 {
@@ -40,7 +40,7 @@ drive_touch :: (env: *void, ev: *MotionEvent) {
     }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     if g_should_call {
         drive_touch(null, null);
     }